The Potential of Zero Liquid Discharge for Sustainable Palm Oil Mill Effluent Management in Malaysia: A Techno-Economic and ESG Perspective

Comments

Author’s Response

The authors have attempted to develop a review on the techno economic and ESG related implications of incorporating ZLD to conventional POME treatment process in Malaysia. The manuscript can be considered for publication after addressing the following matters:

Revise the abstract in a manner that shows the key contribution of this study and the actual content of the review paper.

The authors thank the reviewer for this constructive suggestion. The abstract has been thoroughly revised to more clearly articulate the key contributions of this study.

The revised abstract now explicitly stated the novel framework of analyzing ZLD integration at three strategic points in the POME treatment process, the provision of quantitative techno-economic data, and the in-depth analysis of the Social and Governance dimensions alongside the Environmental aspects.

The revised abstract better reflects the paper's actual content and its significance.

The Techno economic analysis requires refinements. Currently, the assessment is more qualitative and not exhaustive. the authors should  show cases in numerical figures on the relative benefits of ZLD in terms of reducing the operating cost of POME treatment plants

The authors agreed with the reviewer that quantitative data strengthens the analysis. In response, we have:

1.      Add a new Table 3  to provide quantitative comparison of ZLD vs. ponding systems on key metrics such as land use, GHG emissions, water recovery, and payback period. This also addresses suggestion comment by Reviewer 3.

2.      Updated Table 2 with more specific data on energy consumption for some technologies found in literatures.

The section on Social and Governance part of the ESG is very short more  emphasis is given to the environmental feasibility. Hence strengthen those two sections well to make it more appealing

The authors thank the reviewer for highlighting this. The Social (S) in section 3.2 and Governance (G) in section 3.3 have been significantly expanded (as highlighted in yellow in the manuscript).

Comments

Author’s Response

The authors should critically discuss their review approach from a technical-economic perspective.

The authors should update their literature and clearly discuss their contributions and the needs of this manuscript.

For example, the following are included:

Zero liquid discharge and water conservation through water reclamation & reuse of biotreated palm oil mill effluent: a review.

An integrated method for palm oil mill effluent (POME) treatment for achieving zero liquid discharge – A pilot study.

Zero Discharge Performance of an Industrial Pilot-Scale Plant Treating Palm Oil Mill Effluent.

The authors have revised the techno-economic assessment sections to more critically discuss our review manuscript. The authors updated the review around a critical analysis of technologies based on their integration point, technical robustness against POME's challenging characteristics, and economic viability in the Malaysian context. The revision can be found in updated Section 2.2, new Section 2.3 and new comparative Table 3, which provide more information and discussion to address reviewer’s comment.

The authors use too many acronyms; please review them.

The authors apologize for this oversight. A list of abbreviations has been added, and all acronyms are now defined upon first use in the main text. The list has been carefully reviewed to ensure clarity.

The comparative analysis of technologies and management strategies should be supported by tables that include performance data. The current version is very descriptive and does not articulate all sections.

The authors agree with the reviewer. To address this, we have added two new tables:

- Table 2: Provide some detailed with data-driven comparison of specific ZLD technologies including economic assessment.

- New Table 3: offer a quantitative comparison between ZLD and conventional ponding systems.

The authors should present existing global case studies and compare them with the challenges Malaysia faces in achieving this type of development.

The authors have added a new paragraph in the Outlook (Section 4 in line 404) that discusses global ZLD case studies from other industries and extract lesson applicable to Malaysia. The authors also contrast these with the specific challenges faced by the Malaysian palm oil industry, such as the predominance of smallholder mills and seasonal variations in POME production.

Comments

Author’s Response

The article presents a structured and comprehensive review. The authors successfully integrate three critical aspects—technical, economic, and ESG (environmental, social, governance)—to justify the implementation of ZLD technology in the Malaysian palm oil industry.

As a reviewer, I would like to pose the following questions to help the authors further improve and clarify certain aspects of their manuscript:

The article provides an overview of ZLD technologies, but primarily at the level of pilot studies and theoretical models. What are the most compelling examples of industrial-scale implementation of specific ZLD technologies (e.g., MVR or hybrid membrane systems) specifically for POME, and what real-world operational challenges have operators faced during scaling? Are there any data on the long-term (more than 1-2 years) operational stability of such systems?

We thank the reviewer for their positive assessment and valuable questions.

The authors have expanded the discussion on industrial-scale implementations. While full-scale ZLD for POME is still emerging, we now cite and discuss the pilot study by Wang et al. (2015) [2] which treated 100 m³/day, a significant scale-up added in Section 2.2.

Data on long-term operational stability (>2 years) is scarce, but the author have  mention this in Section 4 (line 399) as critical gap for future research

For greater clarity, could you provide a comparative table or quantitative data showing how ZLD systems outperform traditional ponding methods on key metrics? For example:

Percentage reduction in carbon footprint (in kg CO2-eq. per m³ of POME).

Water resource savings (m³/year for a standard-capacity mill).

Reduction in land area occupied (%).

Return on Investment (ROI) or payback period for different scenarios.

The authors agree with the reviewer’s comment and have created a new Table 3 that provide quantitative comparison. The table includes parameter such as:

- Land Area Occupied

- GHG Emissions (kg CO₂-eq/m³ POME)

- Water Recovery Potential

- BOD/COD Removal Efficiency

- Estimated Payback Period

The article focuses mainly on water recovery, while the secondary solid/semi-solid ZLD products are mostly referred to as "bio-fertilizer." However, their composition, volume, and potential issues with subsequent use (e.g., salt accumulation, heavy metals) could be significant. Could you briefly discuss the potential challenges and opportunities associated with managing these final solid wastes within a complete ZLD system?

As suggested by the reviewer, the authors have added a new paragraph in Section 2.3 for briefly discussion on the challenges of ZLD solid residues.

Recovered water and nutrients are a key advantage of ZLD. Are there any real-world data on the quality of these products, backed by independent analysis? For instance, does the recovered water meet standards for reuse in boilers, and are the extracted nutrients safe and effective for direct application as fertilizer without further processing?

We thank the reviewer for this critical question regarding the real-world quality of ZLD products. In response, we have significantly enhanced the manuscript with specific data from pilot and industrial-scale studies in section 2.2.1, 2.2.2, 2.2.3 and 2.3 that validate the quality of both recovered water and nutrients.

Comments

Author’s Response

The manuscript entitled “The Potential of Zero Liquid Discharge for Sustainable Palm Oil Mill Effluent Management in Malaysia: A Techno-Economic and ESG Perspective” presents a timely and relevant review on the integration of Zero Liquid Discharge (ZLD) systems within the Malaysian palm oil sector. The topic is of clear significance to sustainability, water management, and circular economy practices, and it aligns well with the scope of Sustainability.

Overall, the paper is well structured, the discussion is coherent, and the integration of techno-economic and ESG perspectives is commendable. The authors demonstrate a comprehensive understanding of both engineering and policy dimensions of POME treatment. The manuscript provides a valuable contribution by linking ZLD technologies to the Sustainable Development Goals (SDGs), which is rarely done in such a systematic manner.

However, several aspects could be refined to enhance clarity:

The abstract is informative but somewhat lengthy. Please simplify sentences for conciseness and avoid redundancy.

We thank the reviewer for their positive and encouraging feedback, as well as the detailed line-specific suggestions.

We have revised the abstract to improve conciseness and flow, simplified sentences and removed redundancy as suggested.

Line 30 - patronizing its part rephrase to emphasizing its role.

Line 66–74 - The description of section organization could be shortened or placed in a separate paragraph to improve flow.

Line 82–83...as in Figure 1 typically involves…grammatical issue; suggest “as shown in Figure 1, typically involves…”.

Line 99–102 - The statement “waste is sent to polishing pond sedimentation and final quality control” could be supported by an example of discharge standards or COD limits.

Line104 - Edit the reference citation.

Line 105 -108 - Please provide quantitative data on land requirements and retention times to illustrate the inefficiency of ponding systems.

Line 132 - Typo “the high-water content” in “its high water content.”

Line 169–175 - Clarify whether biogas utilization data (28–34 Nm³/m³ POME) comes from industrial or pilot-scale operations.

Line 251-254 - Please clarify whether the MEE schematic is based on simulation or literature data.

Line 294 - “which assisting in protection” in “which assists in protecting".

Line 338–358 Governance section should further discuss barriers to implementation, e.g., financial, regulatory enforcement, and monitoring issues at smallholder mills.

Line 372–377 The digital twin concept is well introduced, but consider citing real pilot examples or ongoing projects in Malaysia or Southeast Asia.

Line 399–403 Collaboration between academia and industry is crucial; please include a recent example.

Line 405-417 The conclusion effectively summarizes the findings but could emphasize limitations and future work. For instance, highlight the need for life-cycle assessments (LCA) or cost-benefit analyses comparing ZLD with advanced anaerobic systems.

Ensure all abbreviations (e.g., MVR, MEE, MD, RO) are defined upon first mention.

We sincerely thank the reviewer for their meticulous reading and specific suggestions. All line-by-line comments have been addressed in the revised manuscript. The changes include:

- Rephrase sentences for better grammar and clarity (Line 30, 294).

- Shorten or moving text for improved flow ( Line 66-74). |

- Add quantitative data and supporting examples ( Line 99-102, 105-108).

- Correcting typos and citation formats ( Line 132, 102).

- Provide clarifications on data sources  (Line 169-175).

- Schematic in figure 3 was based in literature [50].

- Expanding the Governance section with discussion on barriers (Line 391).

- Adding specific examples for real ZLD examples (Line 425-436).

- Adding examples of industry-acadamia collaboration in line 460

- Emphasizing limitations and future work in the conclusion, including the need for LCA (Line 479).

- Ensure all acronyms are defined upon first use.